1. Field of the Invention
This invention relates to grid-modulated plasma switches, generally referred to as CROSSATRON switches, and to the operation of such switches at voltage levels of 100 kV or greater.
2. Description of the Related Art
CROSSATRON switches are grid-modulated plasma switches capable of fast closing speeds like a thyratron, and of rapid opening like a vacuum tube. A sequence of CROSSATRON designs are shown in U.S. Pat. Nos. 4,247,804 issued Jan. 27, 1981 to Harvey, 4,596,945 issued Jun. 24, 1986 to Schumacher, macher et. al. and 5,019,752 issued May 28, 1991 to Schumacher, all of which are assigned to Hughes Aircraft Company, the assignee of the present invention.
The principals of operation of a CROSSATRON switch are illustrated in FIG. 1. The switch is a hydrogen plasma device having four coaxial, cylindrical electrodes disposed around a center axis 2. The outermost electrode 4 is the cathode, which is surrounded by an axially periodic permanent magnet stack 6 to produce a localized, cusp magnetic field 8 near the cathode surface. The innermost electrode 10 functions as an anode, while the next outer electrode 12 is a control grid and the third outer electrode 14 is a source grid.
Secondary electrons produced at the cathode surface are trapped in the magnetic field, and travel in cycloidal ExB orbits (where E is the electric field and B is the magnetic field) around the cylindrical anode 10 due to the radial electric field and the axial component of the magnetic field. The electrons eventually loose their energy via collisions, and are collected by the anode or grids. The long path length of the electrons near the cathode surface enhances ionization of the hydrogen background gas, and reduces the pressure at which the switch operates (compared to thyratrons). The hydrogen pressure in the switch can range from 100 to 700 microns, depending upon the gap spacing between the electrodes and the voltage level. The cathode material is typically molybdenum, and no cathode heater power is required.
The source grid 14 is used to minimize turn-on jitter by maintaining a low level (typically less than 20 mA) DC discharge to the cathode, while the control grid 12 is normally held within about 1kV of the cathode potential. When open, the high voltage in the switch is sustained across the gap between the control grid 12 and the anode 10. The switch is closed by pulsing the control grid to a voltage potential above that of the cathode, thereby building up the density of the plasma 16 so that it diffuses into the gap between the control grid 12 and the anode 10. The result is a low impedance conduction path between the cathode and anode, and a consequent closing of the switch. A high density plasma can be established in the switch, and the rate of current rise to the anode increased, by pre-pulsing the source grid 14 at about 1 microsecond before the closing voltage pulse is applied to the control grid 12.
Current flow through the switch is interrupted by applying a voltage pulse to the control grid 12 that is negative with respect to the potential of cathode 4. The flow of plasma from the production region near the cathode through the control grid apertures is thus blocked, and the switch opens as the plasma erodes from the anode gap. The switch opening time is determined by the plasma erosion time, which is equal to the gap spacing divided by the mean ion diffusion velocity.
The CROSSATRON switch was originally developed as a closing-only switch (U.S. Pat. No. 4,247,804), but was later advanced to a modulator switch capable of high current interruption (U.S. Pat. No. 4,596,945). In U.S. Pat. No. 5,019,752 the cathode was provided with a series of chromium-plated circular perturbations or grooves that extended around the cathode axis. The perturbations increased the effective cathode surface area exposed to the plasma, and thereby reduced the electron emission current density from the chrom surface. A reduction in the switch's forward-voltage drop was attributed to this cathode configuration.
Present CROSSATRON switches have a maximum voltage rating of 50 kV or less. Attempts to raise this voltage significantly have been unsuccessful, due to unreliable voltage standoff and periodic arcing. However, for applications such as plasma-ion implantation, plasma electron hardening, high voltage ion sources, electron guns and klystrode accelerators, the closing and opening capabilities of the CROSSATRON switch should ideally be in the 80-120 kV range. Reliable operation within this range has not been achieved with prior CROSSATRON switches.